Research On NaF-KF-AlF₃ Low Temperature Electrolyte System

Since Hall and Héroult proposed the use of NaF-AlF₃-Al₂O₃molten salt electrolysisto smelt aluminum,the output and consumption of aluminum have grown rapidly,but this method has great disadvantages,such as high energy consumption and large emissions（CO₂）.The shortcomings severely restrict the sustainable development of the aluminum industry.Low temperature aluminum electrolysis can reduce the energy consumption of aluminum electrolysis,and help to realize the industrial application of inert non-carbon electrode aluminum electrolysis technology,thereby achieving zero greenhouse gas emissions.The NaF-KF-AlF₃system has the advantages of lower initial crystallization temperature and better alumina solubility,etc.It is a promising low temperature system for aluminum electrolysis for industrial applications,but LiF and Ca F₂in the NaF-KF-AlF₃electrolyte with low cryolite ratio（CR=（[NaF]+[KF]）/[AlF₃]）When the content is high,the solubility of alumina is greatly reduced,which cannot meet the requirements of aluminum electrolysis.In this paper,the effects of cryolite ratio and electrolyte composition on the liquidus temperature of NaF-KF-AlF₃-LiF-Ca F₂system and the solubility of alumina in the electrolyte（including the solubility and dissolution rate of alumina）are investigated.First,the liquidus temperature of electrolytes with different compositions is studied by the step cooling curve method,and then the experimental temperature of the alumina solubility test is determined on the basis of obtaining the liquidus temperature of the electrolyte,and the oxidation is performed by the rotary corundum sheet method and the LECO RO500 oxygen analyzer The solubility of aluminum in the electrolyte was measured.The dissolution kinetics of alumina in the electrolyte was explored.The main conclusions are as follows:（1）When CR is the same,the liquidus temperature of NaF-KF-AlF₃-LiF-Ca F₂and NaF-KF-AlF₃-LiF-Ca F₂-Al₂O₃system decreases with the increase of LiF and Ca F₂.In the NaF-KF-AlF₃-LiF-Ca F₂system,CR is 1.6-1.8,LiF content is 1-5 wt%,Ca F₂content is 3-5 wt%,the liquidus temperature of the electrolyte is 795.6-851.6℃,which is basically low at 850℃.（2）In the NaF-KF-AlF₃system with a CR of 1.5-1.8,the molecular ratio is constant.When the NaF content is 0-10 mol%,the liquidus temperature of the NaF-KF-AlF₃system increases rapidly;when the NaF content is 35-45 mol%,the liquidus temperature of the electrolyte is low,and the effect of NaF content on the electrolyte The electrolyte in this range is suitable as an electrolyte for low-temperature aluminum electrolysis.（3）The NaF-KF-AlF₃system with a CR of 1.5-1.8 and a NaF content in the range of 0-55 mol%,and a NaF-KF-AlF₃system with CR of 1.6-1.8 and a LiF content of1-5 wt%by regression analysis The liquidus temperature of KF-AlF₃-LiF-Ca F₂system was simulated,and the corresponding fitting equation was obtained.（4）The solubility and dissolution rate of alumina in NaF-KF-AlF₃-LiF-Ca F₂system decrease with the increase of LiF and Ca F₂content,and increase with the increase of temperature and CR.When CR is 1.7,LiF content is 1-5wt%,Ca F₂content is 3-5wt%,temperature is 860℃,the solubility of alumina is 5.46-6.56 wt%,which can meet the needs of aluminum electrolysis.（5）The study found that when alumina is dissolved in the melt,it follows two rate laws,a zero-order reaction and a first-order reaction.When the content of alumina in the melt is lower than the saturation concentration of 60 wt%,the zero-order reaction rate law is observed when the alumina is dissolved;when the content of alumina is greater than the saturation concentration of 60 wt%,the first-order reaction rate law is followed when the alumina is dissolved;The dissolution rate of alumina depends on the dissolution rate of alumina in the electrolyte in the first five minutes.